Process for conversion of para-xylene to high purity dimethyl terephthalate

ABSTRACT

1. A PREPARATIVE PROCESS FOR HIGH PURITY DIMETHYL TEREPHTHALATE HAVING AN ACID NUMBER IN THE RANGE OF 0.01 TO 0.03 AND FREEZING POINT IN THE RANGE OF 140.62 TO 140.63*C BY THE OPERATING SEQUENCE: ESTERIFYING TEREPHTHALIC ACID HAVING 50 P.P.M. 4-CARBPXYBENXALDEHYDE OR LESS AND 160-600 P.P.M. P-TOLUIC ACID WITH METHANOL HAVING 0-5% WATER IN THE RESPECTIVE REACTANT WEIGHT RATIO OF 1.0:3.0 TO 10.0 AT A TEMPERATURE IN THE RANGE OF 200 TO 300*C. AND A PRESSURE IN THE RANGE OF OF 500 TO 50000 P.S.I.G.; PRECIPITATING DIMETHYL TEREPHTHALATE AT A FINAL TEMPERATURE OF 25-40:C. AND A PRESSURE OF 130-250 MM. HG.; SEPARATING DIMETHYL TEREPHTHALATE PRECIPITATE FROM METHANNOL MOTHER LIQOUR; WASHING THE SEPARATED PRECIPIATE WITH METHANOL HAVING 0.5% WATER AND DISTILLING THE WASHED PRECIPITATE TO REMOVE AS OVERHEAD FRACTIONS RESIDUAL METHANOL AND METHYL ESTERS OF P-TOLUIC ACID AND4CARBOXYBENZALDEHYDE LEAVING AS RESIDUAL PRODUCT HIGH PURITY DIMETHYL TEREPHTHALATE.

United States Patent Office Patented Oct. 29, 1974 3,845,100 PROCESS FORCONVERSION OF PARA-XYLENE TO HIGH PURITY DIMETHYL TEREPHTHALATE MeiluteO. Kusak, Chicago, Ill., assignor to Standard Oil Company, Chicago, Ill.No Drawing. Filed Feb. 13, 1970, Ser. No. 11,324 Int. Cl. C07c 69/82 US.Cl. 260-475 B 3 Claims ABSTRACT OF THE DISCLOSURE High purity dimethylterephthalate in high yield is obtained from p-xylene, air and methanolby the following sequence of operating steps:

(a) liquid phase oxidation of p-xylene (99 mole percent purity) with airin the presence of acetic acid solvent and catalysis provided by one ofthe side chain initiators or promoters (acetaldehyde, methyl ketones orsource of bromine) and a cobalt-containing metal oxidation catalystsoluble in acetic acid;

(b) treatment of aqueous solution of the resulting crude terephthalicacid product with hydrogen in the presence of metallic palladium underliquid phase conditions and after separation from catalyst solidterephthalic acid is obtained and separated from water;

(c) esterification of terephthalic acid resulting from (b) with methanolcontaining 5% water in the respective Weight ratio of 1.0:3.0-10.0;

(d) precipitation of DMT from esterification reaction effluent at finaltemperature of 25-40 C. and 130-250 mm. Hg pressure;

(e) recovery of DMT washed with methanol containing 0-5% water; and

(f) distillation of washed DMT.

BACKGROUND OF THE INVENTION To start with p-xylene, air and methanol thepreparation of high purity dimethyl terephthalate (DMT) required forfiber-forming polyester through tranesterification of DMT with a diol,e.g. ethylene glycol, has been conducted by several routes. One routeinvolves partial oxidation of p-xylene to crude p-toluic acid product,esterification of crude p-toluic acid to its crude methyl ester,recovery of methyl p-toluate by distillation followed by oxidation ofmethyl p-toluate to methyl hydrogen terephthalate (mono-methylterephthalate or MMT), recovery of crude MMT as bottoms product bydistillation, esterification of crude MMT by further esterification withmethanol to form an esterification product containing DMT andimpurities, recovery of DMT by distillation of said esterificationproduct to renew methanol, recover DMT and products which can be in partrecycled to one of the foregoing steps. Each of said distillationrecovery of separations represent a separate and distinct purificationtechnique. Such a multi-step process is integrated and is economicallyand commercially useful for high purity DMT but is not sufficientlyversatile and flexible to be adapted to the manufacture of fiber-formingpolyester by elimination of one or more steps and recovering'fibergradequality terephthalic acid as a free acid for 'direct reaction with adiol, e.g. ethylene glycol, for the production of fiber-formingpolyesters.

A second route from p-xylene, air and methanol is economically andcommercially useful for the production of high purity DMT required forfiber-forming polyester manufacture also is a multi-step route. But eachstep is individually conducted and has not without the addition thereofof further processing steps become or likely to be come a fullyintegrated process with the advantages of being modified or adaptedthrough the elimination of one or more of its steps and still usingapparatus from the eliminated steps for the direct reaction of purifiedterephtthalic acid of fiber grade as a reactant with a diol in theproduction of fiber-forming polyesters. This second route is conductedby first converting to terephthalic acid (TA) under liquid phaseoxidation conditions p-xylene in the presence of acetic acid with asource of molecular oxygen and in the presence of catalysis provided byone of the side chain initiators and promoters, (e.g. acetaldehyde,methyl ketones or a source of bromine) and a metal oxidation catalystsuch as cobalt, manganese, cerium or mixtures thereof. By appropriateknown adjustment of ratio of acetic acid to p-xylene, concentrate ofcatalyst compounds, temperature and pressure, such liquid phasecatalytic phase oxidation can be produced to provide high yields (above-95 mole percent) of crude terephthalic acid product as filter cake.Such crude terephthalic acid has just under about 1.0 weight percent oftotal impurities which are in general 4-carboxy'benzaldehyde (4- CBA),p-toluic acid color bodies and color formers. In said crude terephthalicacid there are generally from 0.1 to 0.5 percent 4CBA and 0.01 to 0.05percent p-toluic acid on a weight basis. That crude terephthalic acid isthen separately esterified with methanol used in an amount from 15 to 50moles per mole of crude terephthalic acid at a temperature above 150 C.and under pressure conditions to maintain methanol in liquid phase.Crude terephthalic acid can also be esterified by injecting into a hotbed thereof methanol vapors at a temperature above the boiling point ofDMT to provide a vapor product containing a mixture of DMT and methanolvapors from which crude DMT product is condensed at a temperatureslightly above C. and the condensate, said purified crude DMT can bepurified by recrystallization from ethanol or xylene, the recrystallizedDMT is then melted to remove the recrystallization solvent and then theliquid DMT is frac tionated in two or more rectification towers toremove a low boiling fraction of about 5 to 15% of the liquid DMT feedand the remainder of the feed is further rectified to obtain a highpurity DMT overhead product and to obtain a higher boilingMMT-containing fraction obtaining MMT useful for recycle toesterification. Whereas such a multi-step process has only the DMTpurification and does produce high quality DMT in high yields, thevarious individual steps are not readily adaptable to true integration,the 5 to 15% low boiling impurity fraction tends to diminish potentialDMT yields. Thus steps of this process are not readily eliminated norcan the apparatus used in the eliminated steps be advantageously usedfor obtaining high purity fiber-grade TA product and/ or diolesterification product useful in the manufacture of fiber-formingpolyesters.

Another route proposed for the preparation of DMT from p-xylene, air andmethanol involves reacting these materials in a combination of liquidphase oxidation and esterification in said combination ofoxidation-esterification. Catalysis is provided by a source of cobaltand a source of bromine. The use of a single combinationoxidation-esterification results in somewhat low yields of DMT comingledwith a multitude of other products. By a combination of two or more ofsaid combined oxidation-esterification steps, separated by suitableremoval of intermediate impurities between steps, increased yield ofvery impure ester product is obtained. For example, following the firstoxidation-esterification its process eflluent is distilled andfractionated to remove such products as water, methanol and methylformate, methyl acetate and methylal to provide a feed for the secondcombined oxidation-esterification. The resulting mixture of productscontains about twice as much DMT as is present in a single step process.Such use of two combinations of oxidation and esterification applied too-xylene produced from the second combined oxidation-esterification amixture containing in addition to metal compound impurities (about 1%)the following compounds:

It is evident from the foregoing mixture of compounds, that the use ofp-xylene in place of o-xylene would produce a like mixture of isomericcompounds and recovery of DMT from said mixture could be affected onlythrough the use of several distillationand rectification steps. Fromsuch recovery of DMT there would remain for recycle to one of thepreceding combinations of oxidation and esterification steps for furtherDMT production about 52% of said product components: methyl p-toluate,mono-methyl terephthalate, p-toluic acid, o-xylene, pxylene alcohol andmethanol. Also about 30% of the product component mixture so separatedwould have no use in further production of DMT and would represent aloss of starting materials otherwise being potentially useful for theproduction of DMT.

It is submitted that the foregoing use of two separate stages ofcombined oxidation-esterification would not lend itself to modificationfor eliminating of any one step whereby the apparatus therefrom could beused for obtaining high purity TA of fiber-grade quality or reacting itwith ethylene glycol as the first step in the manufacture offiber-forming polyesters.

A fully integrated process consisting of essential sequence of operatingsteps has now been devised for converting p-xylene and methanol to DMTin high yield impurity. This fully integrated process is not onlyefiicient and unique but its simplicity indicates its uniqueness. Theheart of this integrated process is the simplicity of esterification ofTA and recovery of high quality DMT. Also until the technology andeconomics develop, a portion of the process can be first operated andhigh purity DMT can be produced as an item of commerce and sale tofiber-forming polyester manufacturers until the demand thereforincreases warranting total integration back to p-xylene oxidation. Stilllater when the technology and economics, at the place the process isoperating, advances to the use of high purity TA of fibergrade quality,some of the later steps of integrated process can be eliminated and theapparatus put to other uses.

As used herein and as understood by those skilled in the art ofmanufacturing fiber-forming polyesters, the term high purity as appliedto DMT indicates a DMT product having an acid number (milligrams KOH pergram of DMT product) in the range of 0.005 to 0.05, typically 0.010.03,and a freezing point in the range of 140.62 to 140.63, typically140.623140.625 C.

SUMMARY OF INVENTION The fully integrated process for production of highpurity dimethyl terephthalate (DMT) from p-xylene, air or other sourceof molecular oxygen and methanol consists essentially of the followingsequential operations:

(a) liquid phase oxidation of p-xylene with molecular oxygen as oxidantin the presence of acetic acid as reaction solvent and in the presenceof cobalt-containing catalyst, which can also contain maganese and/orcerium, in combination with side-chain oxidation initiater or promoterwherein the oxidation temperautre, weight ratio of acetic acid top-xylene and concentration of catalyst components are selected in knownmanner to provide a crude terephthalic acid (TA) product having a totalimpurity content indicated by the concentrations of 0.1 to 0.6 percent4-carboxybenzaldehyde (4-CBA) and 0.01 to 0.06 percent p-toluic acid,both on a weight basis;

(b) hydrogen treatment of an aqueous solution of said crude TA in thepresence of liquid water and solid metallic palladium catalyst whereinhydrogen and catalyst are first separated from the treated liquidaqueous solution and then water is separated from crystalline TAproduct;

(c) esterification of said crystalline TA product with methanol of 0-S%water content in an amount of from 3.0 to 10.0 parts thereof per part ofTA on a weight basis at a temperature above 190 C. for example in therange of ZOO-350 C. under a total pressure of 500 to 5000 pounds persquare inch or 34 to 340 atmospheres;

(d)precipitation of DMT from the esterification product of (e) recoveryof DMT Washed with methanol of 0.5% water content; and

(f) distilling washed DMT to recover high purity DMT.

EMBODIMENT OF THE INVENTION A-Liquid Phase Oxidatioin: This proceduralstep can be conducted at temperatures varying from ambient temperatureup to temperatures in the range of 200 to 230 C. depending upon thesystem of catalysis and concentration of catalyst components andp-xylene in reaction solvent. For this oxidation, catalysis is provided,as before mentioned, by oxidation metal-containing catalyst incombination with one of the side-chain oxidation initiators:acetaldehyde, methyl ketone or a source of bromine to provide catalystsystems. Those systems of catalysis are soluble in acetic acid. Themetal portion of the catalyst is preferably used in the form of metalacetate which is conveniently available as metal acetate hydrate, e.g.cobalt acetate tetrahydrate. The metal-containing catalyst is preferablyprovided by the use of the acetate hydrate form of cobalt, manganese,cerium and mixtures thereof. The metal-containing catalyst, when usedwith the aldehyde or ketone organic initiator, have cobalt as the majormetal component, i.e. 50 percent or more of the metal catalyst componentcalculated as cobalt metal.

Liquid phase conditions are maintainable at atmospheric pressure whenreaction temperatures not exceeding 118 C. (boiling point of aceticacid) are used. Such low temperatures are useful for catalysis providedby cobalt acetate tetrahydrate and acetaldehyde, cobalt acetatetetrahydrate and methyl ethyl ketone and combinations of metal andbromine source wherein there is present about equal gram atomic amountsof metal and bromide. For these low temperature oxidations acetic acidsolutions of 2 to 8% metal are used. Acetaldehyde and methyl ketones areused in from 2 to 6 moles per mole of p xylene. The ratio of acetic acidto p-xylene on a weight basis useful is in the range of 3.0 to 10:1.0.The reaction rates are, of course, increased by use of temperatures upto C. achieved by operation at superatmospheric pressure. Such highertemperatures and pressures increase reaction rate both by the effect ofincreased temperature and by higher permissable oxygen concentrations inthe liquid phase reaction medium. These lower temperature oxidations canuse air or commercial oxygen as the source of molecular oxygen oxidant.

As the reaction temperature is increased above 150 C., pressure toobtain liquid phase conditions are increased which provides for theadvantageous use of air to obtain high oxygen concentrations in theliquid reaction medium. As temperature and said oxygen concentrationincreases, the concentration of the metal component and initiator can bedecreased. For example, at 190 to 225 C. total metals concentration inacetic acid of 0.05 to 0.5 weight percent become exceptionally usefuland initiator or promoter concentration in acetic acid can also bereduced. For example, the source of bromine need only provide 0.1 to 0.3weight percent calculated as bromide ion but acetaldehyde and methylketone promoter-initiators are still required in about equi-molarproportions baseed on pxylene. For said higher temperature oxidations2.5 to 5.0 parts of acetic acid per part of p-xylene on a weight basisare used. The catalysis in acetic acid provided by ions of cobalt,manganese, cerium and mixtures thereof with bromine source is preferredbecause p-xylene conversions to TA and TA yields of 95-98 mole percentare obtainable in xylene residence time of 45-60 minutes for continuousoxidations without coproduction of excess quantities of additionalacetic acid (e.g. from acetaldeyhde or methyl ethyl ketone) and thehigher pressures involved to retain acetaldehyde or methyl ethyl ketonein the liquid phase.

As mentioned before, said oxidations can be conducted under their bestsuited conditions of temperature, pressure, catalyst componentconcentration and ratio of acetic acid to p-xylene to provide a crude TAhaving impurity content indicated by 0.1 to 0.5 percent 4-CBA and 0.01to 0.05 percent p-toluic acid on a weight basis. There are also presentin the crude TA other impurities of the color body and color-formertypes. However, by indicating the concentrations of 4-CBA and p-toluicacid impurities the attendant other impurities are to those skilled inthis art thereby indicated.

For the purposes of this oxidation step the use of pxylene of at least99 mole percent pure is required. The impurity is mainly m-xylene(0.05%) with smaller amounts of o-xylene (about 0.2%) and ethylbenzene(0.1-0.2%) whose oxidation products are quite soluble in acetic acidmother liquor (90-100 C.) from which crude TA is recovered.

BHydrogen Treatment: This procedural step can be conducted in knownmanner in the presence of metallic palladium, preferably metallicpalladium dispersed on the surface of activated carbon of low content ofother elements known to poison palladium for example sulfur and copperamong others. Two of the useful techniques use water as a carrier forcrude TA. One technique uses water to carry crude TA sublimed from solidcrude TA. The other technique uses liquid water as solvent for crude TAat temperatures upward from 225 C., e.g. in the range of 250 to 315 C.to carry crude TA in commercially feasible processing quantities. Bothtechniques use only a small amount of hydrogen per pound of crude TAprocessed, for example about 0.01 to 0.1, preferably 0.03 to 0.06, molehydrogen per mole of TA in the crude TA and even then not all thehydrogen is consumed. For such hydrogen treatment techniques when usedfor production of fiber-grade TA (fiber-grade TA is the product reacteddirectly with a diol such as ethylene glycol in the manufacture offiber-forming polyesters), product the manner of conducting TAprecipitation and separation of water from TA crystals after catalystremoval is important. The same controls for TA crystal formation andseparation of water therefrom is not important for the purposes of thisfully integrated combination process as will be later explained becauseTA with 150 p.p.m. up to 6000 p.p.m. p-toluic acid and 5-50 p.p.m. 4-CBAcan be used in the esterification.

Said controls for production of fiber-grade TA are to maximize p-toluicacid retention in water as solute. The 4-CBA impurity is reducedsubstantially by said hydrogen treating techniques to p-toluic acidleaving a small amount of 4-CBA not so reduced. For this process offully integrated sequential process steps, p-toluic acid in the range of150-6000 p.p.m. and non-reduced 4-CBA in the range of 5-10 p.p.m.impurities can be associated with TA recovered because those impuritiesare also esterified with methanol and their resulting esters removed bysimple distillation as seen from the following 760 mm. Hg. boilingpoints of the methyl esters:

'CMethanol Esterification:This esterification step is preferably carriedout preferably with 3 to 10 parts methanol per part of TA attemperatures above 190 C., e.g. ZOO-350 C., and at pressures of 500 to5000 p.s.i.a. (34- 340 atmospheres). It is preferred to conduct theesterification in the presence of a solid catalyst which speed upesterification of TA but do not accelerate ether formation from thealcohol. Excessive ether formation is known to be undesirable. Preferredamong known esterification solid catalysts are metallic zinc, zincoxide, zinc acetate, cadmium sulfate, oxides and acetates of cobalt,copper and manganese and of these metallic zinc and zinc oxide are mostpreferred.

Important and essential to the present inventive integrated process andespecially to the production of crude DMT from which there can be easilyseparated high purity DMT is the use of esterification techniqueswherein the esterification is first conducted in a turbulent zone tokeep TA suspended until MMT forms followed by completion in a quiescentzone which prevents mixing with first zone materials, permits highcarboxyl conversion to ester group and solid catalyst separation. Thesezones provide about equal residence time therein of the reactionmixture. Esterification product is withdrawn from the quiescent zone.The amount of catalyst used based on TA is in the range of 0.1-0.5,preferably 0.2-0.3, weight percent.

The use of 3 to 10 weight parts of methanol per part of TA in theesterification gives high esterification equilibrium (to at least about94-95% conversion of carboxylic acid groups to methyl ester groups) in arelatively short time, e.g. 40-70 minutes and permits retention ofmethyl ester impurities in alcohol mother liquor during DMTprecipitation and recovery. For example 94% conversion of carboxylicacid groups to methyl ester groups provides 11.2% MMT in DMT product onWeight basis. Said amount of methanol provides sufiicient solvent todissolve MMT as well as DMT.

DMT Recovery by Precipitating, Washing and Drying These steps D to Ftogether with esterification conditions are part of the heart of thisinvention. By subjecting the total esterification effiuent to singlestage crystallization to a final temperature of 25-40 C. at 130- 250 mm.Hg. pressure a slurry of DMT in methanol solution of MMT and some ofother esters can be obtained. The recovery of 95-100% methanol from saidmethanol mother liquor can be accomplished in a known manner, forexample, fractionation. Residue from that fractiona tion contains MMTand DMT and thus a major (50- 70%) of the residue can be recycled backto the esterification as a means for minimizing DMT loss. PrecipitatedDMT is recovered by any means for effecting separation of solids andliquids, for example, filtration, classification and centrifugation. Theseparated DMT crystals are washed with 0.05-1.0 parts methanol (15%water) per part DMT by weight to remove adhering moth liquor. The washliquor can be used as part of the methanol for esterification. Thewashed DMT crystalline product is then subjected to distillation toremove adhering methanol and remove methyl p-toluate and methyl ester of4-CBA and recover high purity DMT.

To demonstrate the flexibility of the present inventive process methanolesterification step and subsequent DMT recovery steps can be eliminated.The integrated process is then operated through recovery of TA from thecatalytic hydrogen treatment of aqueous solution of crude TA. In thiscase the controlled TA precipitation and recovery are exercised torecover dry TA product having 5-10 p.p.m. 4CBA and 50-150 p.p.m.p-toluic acid. Such dry TA with ethylene glycol in the respective molarratios of 1.0 to 1.5 to 5.0 are preheated to a temperature above boilingpoint of ethylene glycol (197.5 C.) and under pressure to retain it inthe liquid phase. Suitable temperatures are 200 to 300 C. Preferablythis esterification is conducted and the pressure equal to theautogenetic pressure generated in a closed system by vapors of glycol,esters and by-product water until a clear fluid product is obtained.Such esterification of TA with glycol is advantageously conducted in thesame apparatus used for methanol esterification of TA wherein therearethe turbulent and quiescent zones. The clear fluid esterificationproduct can be subjected to crystallization by eflecting part of thecooling by evaporation of by-product water and glycol resulting indepressurizing to a pressure down to 300 to 500 mm. Hg. Thereafter byfurther heating to 250-280 C. and depressuring down to 0.1 to 0.5 mm. Hgremoval of unreacted glycol and glycol splitting out by polycondensationcan be effected until a polyester of intrinsic viscosity of 0.3 to 0.9is obtained. Such polyesters (0.3-0.5 intrinsic viscosity) are suitablefor film manufacture and (0.5 to 0.9 intrinsic viscosity) are suitablefor fiber manufacture.

Operation of the process of this invention with various unused portionswill be described to further illustrate its practice.

Example 1 First the unique simple preparation of high quality DMT isexemplified using TA having p.p.m. 4CBA and 150 p.p.m. p-toluic acid.

A slurry is prepared from reactants used in the proportions of 830pounds of that TA in 4150 pounds methanol containing 3% water and 0.4pounds catalyst is prepared using fresh methanol, methanol recoveredfrom crystallization mother liquor, methanol Wash liquor from washingcrystalline DMT and methanol recovered from drying washed DMT. Part ofthe catalyst is provided by recycle of MMT and DMT separated as residuefrom re covery of methanol from mother liquor. This slurry iscontinuously heated to 260 C. and pressurized to 1900 p.s.i.g. and thenintroduced into an esterification reactor filled with reaction producthaving a turbulent zone where feed enters and a quiescent zone whereesterification efiluent is withdrawn. After a reactant residence time of40- 50 minutes the fluid effluent withdrawn from the quiescent zonecontains DMT and MMT equivalent to esterification of 95-97% of the totalcarboxylic acid groups to methyl ester groups.

The liquid effluent on an hourly basis, about 5805 pounds, contains 4466pounds methanol, 262 pounds water, 987.5 pounds DMT, 90 pounds MMT andsmall amounts of dissolved metal salt from catalyst and methyl esters of4CBA and p-toluic acid. The liquid efliuent is heat exchanged withpressurized TA slurry in methanol flowing to the esterification reactor.The effluent is partially cooled in this manner. The partially cooledliquid eifluent (about 120 C.) is further cooled to a temperature ofabout 27-28 C. to precipitate crystalline DMT. The resulting methanolmother liquor contains about 50 pounds DMT and all the MMT and methylesters of 4- CBA and p-toluic acid. The crystalline DMT is recovered bycontinuous centrifugal filters wherein DMT cake is washed with methanol(2-3% H O) recovered from methanol mother liquor. This methanol wash isused to make esterification reaction TA slurry. In this way 1102 poundsof wet DMT (937 pounds dry basis) are obtained. This set DMT isdistilled to recover DMT which has an acid number of 0.03 and a freezingpoint of 140.623 C.

Example 2 The process of this invention is next illustrated by way ofstarting with crude TA having about 1.0 weight percent impurities ofwhich 4CBA is the major impurity.

Such a product is obtainable from any of the liquid phase processesbefore described using in their catalyst systems one of the side-chainoxidation initiators (bromine, acetaldehyde or methyl ethyl ketone) incombination with oxidation metal catalysts (cobalt, manganese andcerium) before described.

A slurry of such crude TA in water is prepared containing 20 weightpercent solids at ambient temperature (ZS-30 C.). This slurry is heatedto the temperature of about 5 to 10 C. above the saturation temperature(about 270 C.) and pressurized to a pressure (about 54 atmospheres) atwhich water remains in the liquid phase at such temperature. This hotpressurized solution and hydrogen are combined and are permitted to flowa bed of particulate catalyst of 0.5% palladium on charcoal at thattemperature and pressure. Unused hydrogen is removed from the liquidefliuent from the catalyst bed. The hydrogen free solution is filteredto remove catalyst and then cooled to precipitate TA at the temperatureof 102-105 C. and 2 to 5 p.s.i.g. pressure. The precipitate is separatedfrom liquid aqueous mother liquor, washed with water and dried.

The esterification and DMT recovery processes of Example 1 are repeatedusing the above obtained washed and dried TA in the hourly rate of 830pounds (5.0 pound moles) to produce 937 pounds (dry basis) of highpurity DMT.

Example 3 As feed for preparation of terephthalic acid there arecombined in an hourly basis 366 pounds (3.35 pound moles) p-xylene(99%-pure) 1100 pounds acetic acid (95% acid and 5% water on weightbasis) and cobalt and manganese acetate tetrahydrates to provide 0.94pound total metal (calculated as metal ions) and source of bromine toprovide 1.3 pounds bromine calculated as bromide ion. The acetic acidcomes from solvent recovery later described. Said feed slurry chargedinto an oxidation reactor and compressed air is also introduced into theoxidation reactor containing liquid phase reaction mixture at atemperature of 225 C. and a pressure of about 400 p.s.i.g. Air isintroduced to provide on an hourly basis a slight excess of oxygen overthe 10.35 moles oxygen required for 366 pounds xylene per hoursuificient to provide 12% 0 by volume in the exhaust gas on acetic acidfree basis. The gaseous mixture generated from said liquid phase mixtureby heat of reaction exits oxidation reactor, is condensed and thecondensate is recycled into the oxidation reactor 20.

Fluid reaction elfluent is discharged from the oxidation reactor tocrystallization conducted to a final temperature of -100 C. andatmospheric pressure. The precipitated TA solids are collected (e.g.filtering, classifying or centrifuging) and washed with makeup aceticacid. Mother and wash acetic acid liquors containing catalyst componentsare distilled and fractionated to recover acetic acid (5% water). Wetcrude TA is dried. The dried product, containing about 1.0% impurities,is used in the process of Example 2.

The invention claimed is:

1. A preparative process for high purity dimethyl terephthalate havingan acid number in the range of 0.01 to 0.03 and a freezing point in therange of 140.62 to 140.63 C. by the operating sequence: esterifyingterephthalic acid having 50 p.p.m. 4-carboxybenzaldehyde or less and160-6000 p.p.m. p-toluic acid with methanol having 0-5% water in therespective reactant weight ratio of 1.0:3.0 to 10.0 at a temperature inthe range of 200 to 300 C. and a pressure in the range of 500 to 5000p.s.i.g.; precipitating dimethyl terephthalate at a final temperature of25-40 C. and a pressure of -250 mm. Hg.; separating dimethylterephthalate precipitate from methanol mother liquor; washing theseparated precipitate with methanol having 05% water and distilling thewashed precipitate to remove as overhead fractions residual methanol andmethyl esters of p-toluic acid and 4- carboxybenzaldehyde leaving asresidual product high purity dimethyl terephthalate.

2. The process of claim 1 whereinthe terephthalic acid reactant isobtained by catalytic hydrogen treatment of liquid aqueous solution ofcrude terephthalic acid containing 1.0 percent total impurities,@wherein the impurities consist essentially of color bodies, colorformers and 0.1 to 0.6 percent 4-CBA and 0.01 to0.06 percent p-toluicacid based on the weight of said crude terephthalic acid, in thepresence of a metallic palladium catalyst; separation of treatedsolution from catalyst; precipitation of terephthalic acid crystals fromseparated solution and removal of liquid aqueous mother liquor from theprecipitate wherein said precipitation of crystals and removal of motherliquor are conducted at a temperature within the range of IOQ to 150 C.and drying said recovered precipitate.

3. The process of claim 2 wherein said crude tereph- 10 thalic acid isobtained as the solid product separated from the fluid efl juentobtained by the catalytic liquid phase oxidation of p-xylene of at least99% purity with air. I;

References Cited UNITED STATES PATENTS 3,073,754 1/1963 1 Aroyan et a1.260-475 3,076,019 1/19 3 Ba dwin 260475 6 FOREIGN PATENTS 873,913 8/1961Great Britain 260475 994,769 6/1965 Great Britain 260-525 LORRAINE A.WEINBERGER, Primary Examiner E. J. SKELLY, Assistant Examiner US. Cl.X.R. 260-475 R, 525

"M059 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIQN Patent 3.86.100 D t October 29,197

Inventor) Meilute O. Kusak It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

1 k1 "tranaateriflcstiqn" should be tmwuuuneatmn' 2 2 .'tcrepht-" shouldbe tereph- 4 3 16 "Methyl Acetate 1.5" should be Methyl Formate 1.5 h 2"initiator" should be initiator h 23 "05$" should be o-5$ h 28"Oxidatioi-n" mmm be Oxidation 5 8 "based" should be hand 5 33"(oi-05$)" would be S 73 "5- m should be- 5 -5 m 3 9 Rana first use ofprof-ably? 6 62 "0.05" BhOuld be 0.5 6 63 "moth" should. 1:. mother -7 3"a.bove" ahculd be abovc the T 12 "thereare" ahmld'bc-atfiqi'e are 8 66"160" should be 150 u Signed and sealed this 7th day of January 1975.

ACCESC:

v Z'icCCY Y1. 1 1E303 JR. C. ILQRSITA'L DAN-N Attesting OfficerCommissioner of Patents

1. A PREPARATIVE PROCESS FOR HIGH PURITY DIMETHYL TEREPHTHALATE HAVINGAN ACID NUMBER IN THE RANGE OF 0.01 TO 0.03 AND FREEZING POINT IN THERANGE OF 140.62 TO 140.63*C BY THE OPERATING SEQUENCE: ESTERIFYINGTEREPHTHALIC ACID HAVING 50 P.P.M. 4-CARBPXYBENXALDEHYDE OR LESS AND160-600 P.P.M. P-TOLUIC ACID WITH METHANOL HAVING 0-5% WATER IN THERESPECTIVE REACTANT WEIGHT RATIO OF 1.0:3.0 TO 10.0 AT A TEMPERATURE INTHE RANGE OF 200 TO 300*C. AND A PRESSURE IN THE RANGE OF OF 500 TO50000 P.S.I.G.; PRECIPITATING DIMETHYL TEREPHTHALATE AT A FINALTEMPERATURE OF 25-40:C. AND A PRESSURE OF 130-250 MM. HG.; SEPARATINGDIMETHYL TEREPHTHALATE PRECIPITATE FROM METHANNOL MOTHER LIQOUR; WASHINGTHE SEPARATED PRECIPIATE WITH METHANOL HAVING 0.5% WATER AND DISTILLINGTHE WASHED PRECIPITATE TO REMOVE AS OVERHEAD FRACTIONS RESIDUAL METHANOLAND METHYL ESTERS OF P-TOLUIC ACID AND4CARBOXYBENZALDEHYDE LEAVING ASRESIDUAL PRODUCT HIGH PURITY DIMETHYL TEREPHTHALATE.